The influence of pore‐water advection, benthic photosynthesis, and respiration on calcium carbonate dynamics in reef sands

Abstract

To investigate diel calcium carbonate (CaCO 3 ) dynamics in permeable coral reef sands, we measured porewater profiles and fluxes of oxygen (O 2 ), nutrients, pH, calcium (Ca 2+ ), and alkalinity (TA) across the sedimentwater interface in sands of different permeability at Heron Reef, Australia. Background flushing rates were high, most likely as a result of infaunal burrow irrigation, but flux chamber stirring enhanced pore-water exchange. Light and pore-water advection fueled high rates of benthic primary production and calcification in sunlit surface sediments. In the light, benthic photosynthesis and calcification induced surface minima in Ca 2+ and TA and peaks in pH and O 2 . Oxygen penetration depth in coarse sands decreased from , 1.2 cm during the day to , 0.6 cm at night. Total oxygen uptake (TOU) in dark chambers was three to fourteen times greater than diffusive uptake and showed a direct effect of pore-water advection. Greater sediment oxygen consumption rates were observed in higher permeability sands. In the dark, TA release was not stimulated by increasing TOU because of a damping effect of pore-water advection on metabolic CaCO 3 dissolution efficiency. On a daily basis, CaCO 3 undergoes net dissolution in Heron Reef sands. However, pore-water advection can reverse the CaCO 3 budget and promote CaCO 3 preservation under the most energetic conditions.

Twenty Years of Marine Carbon Cycle Observations at Devils Hole Bermuda Provide Insights into Seasonal Hypoxia, Coral Reef Calcification, and Ocean Acidification

“…Studies of processes such as dissolution of reef CaCO 3 sediments (e.g., Yates and Halley, 2006;Andersson et al, 2007Andersson et al, , 2009Tribollet et al, 2009;Rao et al, 2012;Comeau et al, 2015;Rodolfo-Metalpa et al, 2015;Yamamoto et al, 2015), pore water and alkalinity fluxes from the sediment (e.g., Falter and Sansone, 2000;Santos et al, 2011Santos et al, , 2012aCyronak et al, 2013a,b), and coral bioerosion (e.g., Wisshak et al, 2012;Crook et al, 2013;Enochs et al, 2015;Kline et al, 2015) all contribute to elucidating the complex controls and responses of coral reefs to changes in ocean chemistry.…”

Ocean Acidification Changes Abiotic Processes but Not Biotic Processes in Coral Reef Sediments

“…Coral reef sediments play a key role in the recycling of nutrients within the reef (Rasheed et al, 2002;Werner et al, 2006;Rao et al, 2012). The sediments form upon gradual erosion of carbonates mainly from the coral reef framework, calcifying green alga Halimeda and benthic foraminifers.…”

“…Therefore, sediments act as biocatalytic filters that retain nutrients within coral reefs and therefore contribute to sustain the high biomass and gross primary productivity of coral reefs within oligotrophic environments (Rasheed et al, 2002;Wild et al, 2004;Werner et al, 2006;Rao et al, 2012). Furthermore, coral reef sediments are also places of high photosynthesis rates, which can contribute significantly to the ecosystem's primary production (Werner et al, 2008;Schoon et al, 2010;Rao et al, 2012;van Hoytema et al, 2016). Photosynthesis in sediments is mainly carried out by microphytobenthos (MPB), which is an important food source for higher trophic levels (e.g., fish, sea cucumbers).…”

“…Increased pCO 2 , as from OA, reduces the water column , which acts as the starting point for any changes in carbonate chemistry within the sediments (Andersson and Gledhill, 2013). The consumption of CO 2 by photosynthesis in illuminated surface sediments can increase porewater pH and , thereby causing the abiotic precipitation of carbonates (Schoon et al, 2010;Rao et al, 2012). In turn, CO 2 release from OM remineralization in the dark and in deeper sediments reduces , leading to carbonate dissolution.…”

The impact of sedimentary alkalinity release on the water column CO<sub>2</sub> system in the North Sea

“…Without knowledge of the local in situ hydrodynamics, it is impossible to predict a priori which stirring regime is appropriate for a given site. Here we used two different stirring rates at each station (40 and 80 rpm) to mimic a range of interfacial pressure gradients and solute exchange conditions (Huettel and Gust, 1992;Janssen et al, 2005;Rao et al, 2012). By taking this approach, we are able to discern how sensitive the fluxes at each station are to advective exchange, and thus we get an idea of the uncertainty in our flux estimates for the permeable sites that we visited.…”